Using facebook SDK in webplayer, getting the dread "fatal content error has been detected..." crash.

As soon as I call FB.Init. The same build is working fine on Kongregate. And it will successfully log into Facebook if I run it from the Editor, it is only crashing when it is actually in the Facebook integrated canvas.

How do I find out what the probem is? All I can do now is stick in "return;" at different points, rebuild, reupload and see whether it crashes or not, which sucks. (I could also go back to a backup from a couple of days ago, before any of the Kongregate or Facebook login was working, which would suck even more.)

I looked for the webplayer log where it's supposed to be but there's not even a Unity folder in Library/Logs/. Checking "development build" and "script debugging" doesn't help. I've already read and tried the ideas suggested here and in the forums to fix a "fatal content error." Any more ideas?